Recently, in a hydraulic drive system for construction machines, such as hydraulic excavators and cranes, each equipped with a plurality of hydraulic actuators for driving a plurality of driven members, it is customary to control the discharge pressure of a hydraulic pump in response to load pressures or demanded flow rates, and to arrange pressure compensating valves in association with flow control valves for controlling the differential pressures across the flow control valves by the associated pressure compensating valves, so that the supplied flow rates are steadily controlled when simultaneously driving the hydraulic actuators. Commonly known as a typical example of controlling the discharge pressure of the hydraulic pump in response to the load pressures is load-sensing control.
The load-sensing control system controls the discharge rate of the hydraulic pump such that the discharge pressure of the hydraulic pump becomes higher by a fixed value than the maximum load pressure among the plurality of hydraulic actuators. This control increases and decreases the discharge rate of the hydraulic pump in response to the load pressures of the hydraulic actuators, thereby permitting economical operation.
Since the discharge rate of the hydraulic pump has an upper limit, i.e., available maximum flow rate, the pump discharge rate will be insufficient, when the hydraulic pump reaches the available maximum flow rate in case of simultaneously driving the plural actuators. This is generally known as saturation of the hydraulic pump. If saturation occurs, the hydraulic fluid discharged from the hydraulic pump will flow into the actuator(s) on the lower pressure side in preference to other actuator(s) on the higher pressure side, the latter actuator(s) being hence supplied with insufficient rates of hydraulic fluid, with the result that the plural actuators cannot be driven simultaneously.
To solve the above problem, with a hydraulic drive system as described in DE-A1-3422165 (corresponding to JP-A 60-11706), two drive parts respectively acting in the valve-opening and -closing directions are provided on each pressure compensating valve for controlling the differential pressure across a flow control valve, in place of a spring for setting a target value of the differential pressure across the flow control valve. The discharge pressure of a hydraulic pump is introduced to the drive part acting in the valve-opening direction, and the maximum load pressure among the plural actuators is introduced to the drive part acting in the valve-closing direction. Thus, a control force in accordance with the differential pressure between the pump discharge pressure and the maximum load pressure is caused to act in the valve-opening direction for setting a target value of the differential pressure across the flow control valve. When saturation of the hydraulic pump occurs in the foregoing arrangement, the differential pressure between the pump discharge pressure and the maximum load pressure is reduced correspondingly. Therefore, the target value of the differential pressure across the flow control valve for each pressure compensating valve is also reduced and the pressure compensating valve associated with the actuator on the lower pressure side is further restricted, so that the hydraulic fluid from hydraulic pump is prevented from flowing into the actuator on the lower pressure side with preference. This allows the hydraulic fluid from the hydraulic pump to be distributed corresponding to relative ratios of the demanded flow rates (opening degrees) of the flow control valves and to be supplied to the plural actuators, thereby permitting appropriate simultaneous drive of the actuators.
Such a capability of the pressure compensating valve of reliably distributing and supplying the hydraulic fluid from the hydraulic pump to the plural actuators, irrespective of any discharge condition of the hydraulic pump, is called a "distribution compensating" function in this description for convenience, and hence that pressure compensating valve is called a "distribution compensating valve" in this description.
Meanwhile, when the above hydraulic drive system adopts, as its plural actuators, such actuators as subjected to a relatively large difference between their load pressures, for example, a swing motor and a boom cylinder for respectively driving a swing body and a boom of the hydraulic excavator, and is employed to carry out the combined operation of the swing body and the boom, the following problem has been caused due to a difference in the load pressure therebetween.
When the swing motor and the boom cylinder are driven simultaneously to carry out the combined operation of swing and boom-up for loading earth onto trucks, the above-mentioned function of the distribution compensating valve allows, at the beginning of the combined operation, the flow rate of hydraulic fluid to be distributed to the swing motor and the boom cylinder in accordance with relative ratios of the demanded flow rates of the flow control valve for swing and the flow control valve for boom-up. This will attempt to speed up the swing body responsive to the distributed flow rate. In practice, however, because the swing body has large inertia and the swing motor is subjected to the substantially large load pressure, most of the flow rate supplied to the swing motor is released from a relief valve, and hence not utilized as effective energy. At this time, the pump discharge pressure is so controlled as to become higher by a fixed value than the accelerating pressure of the swing motor on the maximum load pressure side under the load-sensing control. Letting the pump discharge pressure be 250 kg/cm.sup.2, since the pressure necessary for boom-up is on the order of about 100 kg/cm.sup.2, the difference of 150 kg/cm.sup.2 is restricted by the distribution compensating valve associated with the boom cylinder and wasted in the form of heat.
Accordingly, this hydraulic drive system has faced the problems as follows. During the combined operation of swing and boom-up, the system is not economical because of large loss of energy. Furthermore, the flow rate supplied to the boom cylinder is distributed unreasonably in an attempt of carrying out the swing operation simultaneously. This restricts a lift amount of the boom and can cause the boom-up operation to fail with the result that the working efficiency tends to diminish.
It is an object of the present invention to provide a hydraulic drive system for construction machines which can suppress the loss of energy and ensure the operative amount of actuator fluid pressure on the lower load pressure side, when simultaneously driving two hydraulic actuators which are subjected to a relatively large difference between their load pressures.